Conventional pressure sensors using MEMS resonators will be described with reference to FIG. 7.
FIG. 7(a) is a cross sectional view of a pressure sensor disclosed in Patent Literature 1. An vibrator 3, a sacrifice layer 10 and a shell 4 are formed on a silicon substrate by a thin film process, the sacrifice layer is removed by etching, and then an etching fluid-introducing hole in the shell is closed to form a vacuum chamber around the vibrator. Subsequently, the substrate is etched from the backside to make it thinner, and a diaphragm is thus formed.
If pressure is applied to the diaphragm from the backside of the substrate, the diaphragm bends and the vibrator is stressed. If the vibrator is a fixed-fixed beam with both ends being fixed, the axial stress of the beam changes in its longitudinal direction and the resonance frequency of the vibrator changes according to this axial stress. Since the vibrator and shell has a capacitance between them, excitation of the vibrator and detection of the frequency change are electrically operated. As a result, the change of the axial stress can be measured based on the change of the resonance frequency of the vibrator, and a force given to the diaphragm, i.e. the pressure, can be measured based on the change of the axial stress.
In manufacturing the pressure sensor of Patent Literature 1, the vibrator, vacuum chamber and shell are formed on the front side of the silicon substrate, and then it is required to overlay an etching mask on the backside of the silicon substrate and perform etching while protecting the front side in order to form the diaphragm. It is therefore difficult to control the thickness of the diaphragm with high accuracy, if the diaphragm is designed to be extremely thin and the desired thickness is by far thinner than the thickness of the substrate.
FIG. 7(b) shows a cross sectional view of a pressure sensor disclosed in Patent Literature 2. A vacuum chamber is formed around an vibrator 14 by a thin film. The difference from the pressure sensor of Patent Literature 1 is that a structure 13 with the outermost surface of a polysilicon layer 11 functions as a diaphragm. This pressure sensor measures pressure by converting pressure that is applied on the surface of the diaphragm 13 to a stress of the vibrator 14, electrically detecting the change of the resonance frequency of the vibrator 14, and converting the detected electrical signal to the pressure. The sensitivity to pressure depends on the rigidness of the diaphragm, i.e. the thickness of the polysilicon layer. Since the diaphragm is not formed by etching the substrate, the thickness of the diaphragm is easier to control in the pressure sensor disclosed in Patent Literature 2 than of Patent Literature 1.